The present invention relates to a key for use in a cylinder lock with a rotatable key plug having a profiled key hole or keyway, said key comprising:                an elongated key blade with a relatively wide lower portion and a relatively narrow upper portion, there being a step-like transition region between the wider lower portion and the narrow upper portion,        said key blade having a longitudinal profile groove located adjacent to said step-like transition region and extending along at least a portion of the length of the key blade, with an inner wall of said groove being substantially parallel to a side surface of said lower portion of said key blade,        said longitudinal profile groove having an undercut portion adjacent to and inside a ridge portion of the key blade, the outside of said ridge portion forming a part of said side surface of the key blade in said relatively wide lower portion up to said step-like transition region, and the inside of said ridge portion facing said inner wall of said groove.        
Such a key with an undercut groove is previously known, e.g. from U.S. Pat. No. 5,715,717 (Widén) or U.S. Pat. No. 5,640,865 (Widén). Such keys have proven to be very useful in that they provide an improved security. The key profile is quite distinguished from conventional keys, and it is rather difficult to copy such keys. Moreover, they permit a great variation of the cross-sectional profile, which is a great advantage.